There are cases where it is necessary to adjust output amplitude without changing the iput digital waveform data in arbitrary signal generators (AWGs) and audio signal sources, etc., with which digital waveform data are input to a digital analog converter (D-A converter; DAC) and analog signals are generated.
FIG. 1 is a conventional circuit with which fixed resistor 102 and relay 103 are connected to the output of a D-A converter (not illustrated) via analog input 101, and the relay switch is switched and controlled by relay control circuit 106 in order to adjust the amplitude of analog output. The desired amplitude is obtained from analog output 104 based on the combination in which the relay switches are connected and disconnected. However, there is a problem with this structure in that it is necessary to use high-precision resistors and therefore, its cost is high. Moreover, many resistors and relays are needed for precise control of resolution and therefore, there are limits to the capability of such a system. Furthermore, there is a disadvantage in that because operating speed of the relays is slow, high-speed amplitude control is not possible. In addition, there is a problem in that the parts are large and miniaturization of the circuit is impossible.
FIG. 2 is another conventional circuit which, using a digital filter as digital multipiler 202, the amplitude of the analog output of D-A converts is adjusted by multiplying a proportional coefficient by the input digital signals and inputting to D-A converter 201. However, by means of this structure, there is a problem since there are limits to the operating speed of the digital multiplier, and high-speed operation is impossible.
FIG. 3 is yet another conventional circuit that uses D-A converter (multiplying-type DAC) 301 with which the reference voltage for D-A conversion can be adjusted to generate reference voltage Vref by D-A converter 302 and add it to D-A converter 301. However, there is a problem in that when this type of structure is used, D-A converter 301 that can use reference voltage is necessary and reference voltage Vref is an analog value and noise can easily increase. Inserting an analog filter between D-A converter 301 and D-A converter 302 has been considered to prevent this noise from being generated, but there is a disadvantage in this case in that high-speed control of DAC output amplitude is not possible.